We propose to characterize marrow-dependent (M) cells, which function to protect mice against Friend virus leukemia, and T suppressor cells, which mediate immune suppression by Friend virus. The precise mechanisms and immunogenetic requirements for suppression by T suppressor cells will be investigated. The ability of Friend virus to suppress lymphocyte mitogenesis in vitro can be used to evaluate the numbers of T suppressor cells in lymphoid tissues. This assay will be used to study such cells in aging mice prone to tumor development and autoimmune mice. Leukemic mice will be irradiated and grafted with H-2 compatible marrow cells from donors resistant to leukemia as a model for treatment of acute leukemia.